The invention relates to a diaphragm for defining a radiation beam, particularly an X-ray beam, said diaphragm being more particularly intended for the positioning in front of the light amplifier of an X-ray unit.
An X-ray unit comprises an X-ray source transmitting a divergent beam in the direction of a receiver, called an image intensifier. The patient is positioned between the source and the image intensifier. It is known to adjust the width of the beam by means of a diaphragm placed between the source and the patient. This makes it possible to limit the radiation exposure of the patient to what is absolutely necessary for carrying out the diagnosis. Moreover, the image intensifier has a circular reception surface, whose diameter is determined as a function of the maximum aperture of the beam. This beam can be 40 cm. When the beam aperture is limited by the aforementioned diaphragm to smaller dimensions, the corresponding X-ray image only forms on part of the image intensifier surface, whereby said part is always centred. Under these conditions, the image appears in the centre of a stray radiation halo, which is prejudicial to its sharpness and makes it more difficult to interpret. It is therefore important to eliminate that part of the stray radiation surrounding the image, by placing an annular absorbing structure above the unused part of the reception surface of the image intensifier.